Hi,
OAT groundschool is actually very good, as long as you work hard you'll get through in the 6 months. some elements of the syllabi are tricky so I assume as a foreign student your english will be perfect - otherwise you may find areas more difficult than they should be !
The teachers in ground school are all ok, despite one or two notable exceptions, but think it's like that anywhere. If you're modular then look at other cheaper options but for integrated OAT groundschool is good.
As for students, well a healthy mix, not that many foreign students from the more desirable parts of europe that you mentioned but plenty from the undesirable parts of the non european med, but they didn't bother us too much when I was there.... good luck
sb